Hi Zeeshan,
If you want to restrict the access to a particular workspace, then you can do this via permissions:
https://www.gooddata.com/docs/cloud/manage-organization/manage-permissions/. When the proper permission is in place, for example the user do not have access to the workspace, they will get an error stating so when they try opening the link.
Alternatively, you may share dashboards between certain users, but please note that this is not a security feature:
https://www.gooddata.com/docs/cloud/create-dashboards/share-dashboards/
If you wish to customize the error message shown by GoodData, I am afraid that this is not possible out of the box. You may be able to capture the event of the wrong permission on your app and redirect the user to a custom page with more info, so to speak, but I would not be able to advise further on this implementation.